“It’s what scares you about training camp; it’s what scares you about the preseason,” 49ers head coach Kyle Shanahan said. “You kind of just want the game to end when those things start happening because you want those guys when the season starts. But we also need to practice and get better. So it’s just something you have to deal with.”

Of that group, Breida, the only player who didn’t exit to the locker room, was only briefly examined by the medical staff. And Harold briefly returned to game before he was forced to exit again.

Shanahan said Kittle and Breida would miss “some time” and he didn’t expect them to practice next week or play when the 49ers visit Houston on Aug. 18.

Kittle, whose strong training camp has suggested he’s ready to build on his promising 2017 rookie season, had his right shoulder examined on the sideline. He then headed to the locker room with his right arm pinned to his midsection. Kittle stayed on the ground for a few moments after he was hit trying to corral a deep pass over the middle from Garoppolo on the 49ers’ opening drive.

For Smith, his hamstring injury was the latest medical setback of his brief 49ers tenure.

After signing a five-year deal last year, Smith missed the 2017 season with a torn pectoral sustained early in training camp. After missing the first four training-camp practices this year with an unspecified lower leg injury, he said this week he was eager to see his first game action Thursday since January 2017 when he was with the Raiders.

His long-awaited return was short-lived: Smith was injured on the game’s opening drive and was in the locker room by the time the Cowboys finished it with a touchdown.

In his only series, Dallas quarterback Dak Prescott directed a 75-yard march capped by a 30-yard pass to wideout Michael Gallup. On the score, Gallup beat cornerback Jimmie Ward, who was replacing Richard Sherman (hamstring), on a streak down the right sideline.

As expected, most of the 49ers’ biggest names barely broke a sweat.

On offense, rookie right tackle Mike McGlinchey and right guard Mike Person were the only starters to play after their nine-play first drive ended with a punt. Garoppolo completed 3 of 6 passes for 34 yards and running back Jerick McKinnon (three carries, minus-4 yards) received woeful run blocking.

On defense, tackle DeForest Buckner, nose tackle Earl Mitchell and safety Jaquiski Tartt exited after one series. Many other starters played through the first quarter, including inside linebacker Reuben Foster, who sprinted about 20 yards downfield to break up a sideline pass to wideout Tavon Austin.

Backup quarterback C.J. Beathard rebounded from a miserable start in which he completed 2 of 8 passes for 51 yards with an interception in the red zone. Beathard finished by completing 8 of 12 passes for 130 yards, including a 53-yard rainbow to rookie wideout Dante Pettis, who easily slipped behind the Cowboys’ coverage.

With the 49ers trailing 21-16 at the two-minute warning, third-string QB Nick Mullens led a six-play, 77-yard drive he capped with a 7-yard pass to rookie wideout Richie James with 18 seconds left. Mullens added a two-point conversion pass to tight end Wes Saxton.